This invention relates to circuit protecting devices and particularly those which employ gas filled tubes which ionize in the presence of excessive voltage conditions thus acting as low impedance shunts to protect the equipment constituting the load.
A power line application of this type of circuit protection which has been developed by the applicant provides a shunt protection circuit across the load, the protection circuit comprising the series combination of a three element gas tube and a thermal circuit breaker.
For short duration surges the normally-closed thermal breaker switch is inactive while the overvoltage tube functions in conventional manner to short-circuit the effects of the transient surge.
In the event of a prolonged surge, the thermal breaker switch opens thereby disconnecting the gas tube from the lines and the load. This arrangement offers potential improvements over prior art power line protectors employing thermal breakers connected in series between the source and the parallel combination of gas tube and load circuit.
However, while the function of the shunt thermal breaker prevents burnout of the gas tube, it leaves the equipment unprotected until the thermal breaker resets.
One proposal for dealing with this particular condition is to include a fuse in the power line in series between the source and the load so that an excessive current condition occurring after the thermal breaker opens will cause the fuse to open thus preventing equipment burnout. Of course, the fuse burnout necessitates human intervention to restore normal conditions. Furthermore, some difficulties have been experienced with this arrangement in that occasionally the fuse opens during those transient conditions which are properly bypassed by the gas tube. As a consequence the load becomes disconnected from the line notwithstanding the shunt protection circuit is fully operable.